If a merchant says an offer expires on a certain day, when does it expire? At 12:00am on the morning of that day or at 11:59pm that evening?

I always thought that it would be at the end of the day specified, but I had one merchant this month where they said the offer expired on January 31 but it turned out it expired at the start of January 31 and not the end. I can't remember which merchant it was now but I was surprised and it got me worried that more merchants might be doing that and I not know. Has anyone else had this issue?

If a merchant says their offer expires on a date, do you set the expiration in your database for that date or the day before? Enquiring minds want to know ...